In a magneto-tunnel junction (MTJ) which has two ferromagnetic material layers supporting a thin insulating layer by sandwiching the thin insulating layer, the tunnel resistance changes depending on the angle of mutual magnetization in each of the ferromagnetic material layers. There is what is called an MRAM as a semiconductor storage device in which an MTJ utilizing this tunnel magneto-resistance (TMR) effect is used as a magnetic storage element (a TMR element) and a plurality of TMR elements are arranged as memory cells, for example, in a matrix manner. It is general practice that a word line and a bit line for performing data writing into each TMR element and reading therefrom, and a selection transistor for selecting a desired memory cell are provided as this MRAM. For example, conventional MRAM is described in U.S. Pat. No. 6,815,783, U.S. Pat. No. 6,891,241, and U.S. Pat. No. 6,992,923.
In this MRAM, during data writing, a current is caused to flow through the word line and the bit line by turning the selection transistor off, and the magnetization direction of the ferromagnetic material layer (free layer) of the TMR element is determined by a composite magnetic field generated from the current. During data reading, a current is caused to flow through the bit line by turning on the selection transistor of the relevant memory cell and on/off states are read on the basis of a difference from a reference current value.
Although conventional MRAMs have the advantage that high-speed switching is possible in a non-volatile memory, it has been pointed out that conventional MRAMs are inferior to SRAMs and DRAMs in terms of power consumption because in principle, several milliamperes are required as a current which is caused to flow through the word line and the bit line during data writing. At present, it is considered that it is possible to suppress the current during writing to 1 mA or so by using a structure in which the magnetic flux density is increased by narrowing a design rule to 0.18 μm and besides a clad layer covering these interconnects with a magnetic material is formed, whereby magnetic fluxes can efficiently pass the TMR element. However, in order to further reduce power consumption, it is necessary to bring the interconnects nearer to the TMR element or to apply the free layer with a low inverted magnetic field, and no other effective methods have not been found out. On the other hand, because in association with requests for further miniaturized designs of semiconductor devices, inverted magnetic fields of the TMR element tend to increase abruptly, it becomes more difficult to reduce the current during writing.